Among methods of electroplating non-conductive plastic moldings to form a decorative coating, a widely used method comprises, in succession, degreasing, etching, optionally neutralizing and pre-dipping, then applying a catalyst for electroless copper plating by using a colloidal solution containing a tin compound and a palladium compound, and optionally activating (performing accelerator treatment), followed by electroless copper plating and electroplating.
A frequently used electroless copper plating solution in such a plating method contains a reducing agent that has high reducing power, such as paraformaldehyde. When this type of electroless copper plating solution is used, copper is deposited in the initial plating stage on a highly catalytic palladium portion of a tin-palladium colloidal coating attached as the catalyst. Then, the copper deposition continues due to the reducing action of the reducing agent with high reducing power, and a copper layer is formed not only on the palladium portion but also in the transverse direction. As a result, copper is also deposited to form a layer on an inherently non-catalytic tin portion so that a bridge deposit is formed, and a spongy coating is likely to be formed.
When electroplating is conducted on the electrolessly plated surface having the bridge deposit, pit-like fine agglomerates (called “stardust”) partially deposit in a large number, thereby tending to render the plated layer uneven. This layer often shows an inferior appearance compared with a plated layer formed on a metallic substrate.
To prevent the creation of such inferior appearance, a method of forming a decorative coating of excellent appearance is known, the method comprising applying a catalyst for electroless plating to a plastic molding, forming a coating having excellent conductivity with no bridge deposits by using an electroless copper plating solution containing a saccharide having relatively weak reducing power as a reducing agent, and performing electroplating directly on the resulting coating (Patent Literature (PTL) 1).
However, even if this conductive-coating bath is used, it is difficult to uniformly electroplate a large non-conductive plastic molding, and thus, there is a high demand for a more excellent conductive-coating bath.